(1) Field of the invention
The present invention relates generally to an apparatus for converting an analog signal into a corresponding digital signal, in which a reference voltage level for determining an analog-to-digital conversion is controlled in accordance with a digital output signal thereof and having a hysterisis characteristic in which the input voltage is compared to a first set of threshold voltages as the input voltage increases and is compared to a second set of threshold voltages as the input voltage decreases.
(2) Background of the Art
A parallel comparison type analog-to-digital converter (hereinafter referred to as an A/D converter) has commercially been available on a market.
In the comparison-type A/D converter, a resistor ladder having series connected resistors for dividing a supplied reference voltage to provide a plurality of reference voltages receives a selected value of a counter for a reference clock decoded by means of a decoder. The reference voltage to be compared is selected and outputted to a comparator in which the reference signal is compared with an input analog signal. When both signals coincide with each other, a gate signal is supplied to a register. The value of the counter is held and outputted as a digital output value.
However, such an A/D converter as described above is appropriate to monolithic conditions. An output digital value is not stable for the input signal in range of a comparison voltage.
For example, in a digital water temperature meter for a vehicle, the analog signal from the sensor is processed by means of a processing circuit and the water temperature is finally indicated through an turned on or turned off of a segment such as a liquid display.
In the processing circuit, the A/D converter is indispensable for the digital display. The digital display unit displays a continuous analog signal as the turn-on and turn-off of a segment of the digital display unit, i.e., a discontinuous value. If the analog signal is minutely varied in the vicinity of a level at which the segment is switched to the turn on or turn off state, the turn on-and-off of the segment is repeated and difficult to view. Therefore, an unpleasant feeling afflicts the user.
To prevent such a problem as described above, the number of segments need to be increased, i.e., the resolution of the A/D converter needs to be increased. If the digital display unit becomes comparable to the analog display unit, the circuit becomes complex at a remarkable cost increase is involved.
If a certain characteristic is provided in the display mechanism or in the A/D converter, flickers in the output value due to noise can be prevented.
Some A/D-conversion and digital display systems having such a characteristic have been proposed.
The A/D converter converts the input signal from a sensor, such as a thermistor into the digital value. A digital delay circuit updates the output value when this digital value repeats the same value a particular number of times, or carries out the output processing such that an average value for the particular number of times is derived and outputted. A decoder decodes the output value of the digital delay circuit for the display segment and a driver drives and displays a display unit.
In the later example, the flickers (or chattering) of the turn-on-and off of the segment through the processing of the digital delay circuit is prevented. However, a favorable response characteristic to a change in the sensor output cannot be achieved. Therefore, its application range is limited.
In another example, the sensor input signal is converted into the digital value by means of a high resolution A/D converter. If a change in the digital value from the A/D converter falls in a particular range, the value outputted to the decoder is not updated.(Refer to a New Version Automotive Engineering Manual Volume 7, page 62, published in Japan by Automotive Engineering Society, May 31, 1983).
For example, in a case of a 2-bit A/D conversion, relationship between the sensor input level and 2-bit digital output value is established. If the digital output is used to drive the digital display unit by means of the decoder and driver, the flickers of the turn on-and-off of one segment of the display unit due to minute fluctuations of the sensor input signal can be prevented. The response characteristic is also improved.
However, in the above-described latter example, in the way as at least three-bit A/D converter is needed for the 2-bit display resolution, at least an A/D converter having an accuracy double the resolution of the display unit becomes necessary. Accordingly, its circuit scale is substantially double or more. In addition, if the system for the A/D conversion and display function is constructed in the hardware such as a custom IC, its circuit scale becomes large and uneconomical.